1. Field of the Invention
This invention relates to the field of thermal efficiency in steam turbine generators. More particularly, it relates to an improved apparatus and method for utilizing steam-to-steam reheat drains.
2. Description of the Prior Art
Virtually all nuclear steam turbine generators, operating under slightly wet or low super-heated initial steam conditions , incorporate steam-to-steam reheat to improve thermal performance and reduce blade erosion. The early reheat units, for example, were single stage designs in which throttle steam was used to reheat high pressure exhaust steam that had been dried in a moisture separator. Subsequently, two-stage reheating was adapted in which the first stage, which receives the dried steam from the moisture separator, utilizes partially expanded extraction steam from the high pressure turbine element as the heating source. A second stage, which follows the first, utilizes throttle steam as the heating source.
Recent strides have been made in turbine design to include higher initial pressures and temperatures, as well as the addition of reheat features. This has brought about an increase in the number of heaters that can be justified in the turbine cycle. The increases in the fuel cost that have justified the higher pressures and temperatures for the turbine have also made it economical to design for higher outlet water temperature by including separate sections that utilize the superheat of the steam. Further, it has been found economical to include drain cooling sections in the heater that subcool the condensate after it has been condensed on the outside of the tubes to a temperature of within 6.degree. C. (10.degree. F.) of the entering feedwater.
In the latter 1970's, research showed that high temperature, high pressure, breeder reactor plants could also beneficially use steam-to-steam reheating and that extraction steam was the optimum steam supply for both single and two stage reheat applications.
At the present time, design practice is to discharge the drains , which are composed of a mixture of condensed steam and scavenging steam, from the high pressure reheater to the highest pressure feedwater heater. The drains from the low pressure reheater of a two-stage reheater design are discharged to either the highest pressure feedwater heater or, when there are concerns about adequate drainage, to the next lower pressure feedwater heater. The drains leaving the high pressure reheater are considerably hotter than the feedwater leaving the highest pressure feedwater heater. The difference can be as much as 55.degree. C. (100.degree. F.) at rated load, and in excess of 140.degree. C. (250.degree. F.) at 25% of rated load. Because the pressure of the reheater drains is higher than the heater extraction pressure, the drains are throttled down to the feedwater pressure prior to heat exchange. This results in a significant loss in thermal efficiency.
It has been suggested that the high pressure reheater drains be sent to a pump that would discharge to the exit of the highest pressure feedwater heater. This method, however, suffers from a major drawback in that it requires an additional pump as well as the difficulty of avoiding cavitation due to either insufficient net positive suction head (NPSH) during steady-state conditions or flashing during transients. There is the further problem of the disposal of reheating scavenging steam that is used to enhance the reheater tube bundle reliability.
Despite the advances reflected by the current state of the art, there always exists the need for new methods and apparatus to increase the thermal efficiency of steam generation systems while avoiding operational and maintenance problems. Accordingly, there exists the need for a method and apparatus that can reduce fuel costs or more efficiently utilize steam within a steam turbine generator power generation system.